Multiplying digital-to-analog converter with pre-sampling and associated pipelined analog-to-digital converter

ABSTRACT

A multiplying digital-to-analog converter (MDAC) includes an operational amplifier, a sampling capacitor circuit, a pre-sampling capacitor circuit, and a switch circuit. During a sampling cycle, the switch circuit connects a pre-defined voltage and reference voltages to the pre-sampling capacitor circuit, disconnects the pre-sampling capacitor circuit from an input port of the operational amplifier and the sampling capacitor circuit, disconnects an output port of the operational amplifier from the sampling capacitor circuit, and connects a voltage input to the sampling capacitor circuit. During a conversion cycle, the switch circuit connects the pre-sampling capacitor circuit to the sampling capacitor circuit, disconnects the pre-defined voltage and the reference voltages from the pre-sampling capacitor circuit, connects the pre-sampling capacitor circuit to the input port of the operational amplifier, connects the output port of the operational amplifier to the sampling capacitor circuit, and disconnects the voltage input from the sampling capacitor circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/925,270, filed on Oct. 24, 2019 and incorporated herein by reference.

BACKGROUND

The present invention relates to conversion between an analog signal anda digital signal, and more particularly, to a multiplyingdigital-to-analog converter (MDAC) with pre-sampling and an associatedpipelined analog-to-digital converter (ADC).

Analog-to-digital converters (ADCs) are employed in a variety ofelectronic systems. Such systems demand cost-effective ADCs that canefficiently convert an analog input signal to a digital output signalover a wide range of frequencies and signal magnitudes with minimalnoise and distortion.

An ADC typically converts an analog signal to a digital signal bysampling the analog signal at pre-determined sampling intervals andgenerating a sequence of binary numbers via a quantizer, wherein thesequence of binary numbers is a digital representation of the sampledanalog signal. Some of the commonly used types of ADCs include FlashADCs, pipelined ADCs, successive approximation register (SAR) ADCs, etc.Of these various types, the pipelined ADCs are particularly popular inapplications requiring higher resolutions. The typical pipelined ADCsuse switched capacitor circuits to add or subtract charges and activecircuits like operational amplifiers to realize multiplication, they arehighly susceptible to component mismatch like capacitor mismatches andcircuit imperfections like finite amplifier gains. Furthermore, thetypical pipelined ADC may employ a high gain and high speed operationalamplifier which has high power consumption and requires backgroundcalibration. Thus, there is a need for an innovative low-power pipelinedADC without background calibration.

SUMMARY

One of the objectives of the claimed invention is to provide amultiplying digital-to-analog converter (MDAC) with pre-sampling and anassociated pipelined analog-to-digital converter (ADC).

According to a first aspect of the present invention, an exemplarymultiplying digital-to-analog converter (MDAC) is disclosed. Theexemplary MDAC includes an operational amplifier, a sampling capacitorcircuit, a pre-sampling capacitor circuit, and a switch circuit. Theoperational amplifier has an input port and an output port. The switchcircuit is arranged to control interconnection between the operationalamplifier, the sampling capacitor circuit, and the pre-samplingcapacitor circuit. During a sampling cycle of the MDAC, the switchcircuit is arranged to connect a pre-defined voltage to the pre-samplingcapacitor circuit, connect a plurality of reference voltages to thepre-sampling capacitor circuit, disconnect the pre-sampling capacitorcircuit from the input port of the operational amplifier, disconnect thepre-sampling capacitor circuit from the sampling capacitor circuit,disconnect the output port of the operational amplifier from thesampling capacitor circuit, and connect a voltage input of the MDAC tothe sampling capacitor circuit. During a conversion cycle of the MDAC,the switch circuit is arranged to connect the pre-sampling capacitorcircuit to the sampling capacitor circuit, where a configuration ofconnection between the pre-sampling capacitor circuit and the samplingcapacitor circuit depends on a quantization result of the voltage input,and is further arranged to disconnect the pre-defined voltage from thepre-sampling capacitor circuit, disconnect said plurality of referencevoltages from the pre-sampling capacitor circuit, connect thepre-sampling capacitor circuit to the input port of the operationalamplifier, connect the output port of the operational amplifier to thesampling capacitor circuit, and disconnect the voltage input from thesampling capacitor circuit.

According to a second aspect of the present invention, an exemplarypipelined analog-to-digital converter (ADC) is disclosed. The exemplarypipelined ADC includes a plurality of stages and a combining circuit.The stages are arranged to generate a plurality of digital outputs,respectively. The combining circuit is arranged to combine saidplurality of digital outputs. At least one of said plurality of stagesincludes a quantization circuit and a multiplying digital-to-analogconverter (MDAC). The quantization circuit is arranged to generate aquantization result of a voltage input of said at least one of saidplurality of stages, wherein a digital output of said at least one ofsaid plurality of stages depends on the quantization result of thevoltage input. The MDAC includes an operational amplifier, a samplingcapacitor circuit, a pre-sampling capacitor circuit, and a switchcircuit. The operational amplifier has an input port and an output port.The switch circuit is arranged to control interconnection between theoperational amplifier, the sampling capacitor circuit, and thepre-sampling capacitor circuit. During a sampling cycle of the MDAC, theswitch circuit is arranged to connect a pre-defined voltage to thepre-sampling capacitor circuit, connect a plurality of referencevoltages to the pre-sampling capacitor circuit, disconnect thepre-sampling capacitor circuit from the input port of the operationalamplifier, disconnect the pre-sampling capacitor circuit from thesampling capacitor circuit, disconnect the output port of theoperational amplifier from the sampling capacitor circuit, and connect avoltage input of the MDAC to the sampling capacitor circuit. During aconversion cycle of the MDAC, the switch circuit is arranged to connectthe pre-sampling capacitor circuit to the sampling capacitor circuit,where a configuration of connection between the pre-sampling capacitorcircuit and the sampling capacitor circuit depends on the quantizationresult of the voltage input, and is further arranged to disconnect thepre-defined voltage from the pre-sampling capacitor circuit, disconnectsaid plurality of reference voltages from the pre-sampling capacitorcircuit, connect the pre-sampling capacitor circuit to the input port ofthe operational amplifier, connect the output port of the operationalamplifier to the sampling capacitor circuit, and disconnect the voltageinput from the sampling capacitor circuit.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a multiplying digital-to-analogconverter (MDAC) with pre-sampling according to an embodiment of thepresent invention.

FIG. 2 is a circuit diagram of an MDAC with pre-sampling according to anembodiment of the present invention.

FIG. 3 is a diagram illustrating the principle of the proposed MDACdesign with pre-sampling according to an embodiment of the presentinvention.

FIG. 4 is a diagram illustrating a transfer curve of the MDAC shown inFIG. 2.

FIG. 5 is a diagram illustrating an equivalent circuit of the MDAC shownin FIG. 2 that operates during the sampling cycle.

FIG. 6 is a diagram illustrating an equivalent circuit of the MDAC shownin FIG. 2 that operates under a condition of (V_(ip)−V_(in))>V_(ref)/4during the conversion cycle.

FIG. 7 is a diagram illustrating an equivalent circuit of the MDAC shownin FIG. 2 that operates under a condition of−V_(ref)/4≤(V_(ip)−V_(in))≤V_(ref)/4 during the conversion cycle.

FIG. 8 is a diagram illustrating an equivalent circuit of the MDAC shownin FIG. 2 that operates under a condition of −V_(ref)/4<(V_(ip)−V_(in))during the conversion cycle.

FIG. 9 is a diagram illustrating common-mode suppression achieved by theproposed MDAC with pre-sampling according to an embodiment of thepresent invention.

FIG. 10 is a diagram illustrating a differential amplifier without atail current source according to an embodiment of the present invention.

FIG. 11 is a circuit diagram of an MDAC with pre-sampling andCLS-assisted operational amplifier according to an embodiment of thepresent invention.

FIG. 12 is a diagram illustrating an operation of the DAC-subtract-gainfunction performed by the MDAC shown in FIG. 11 that includes a samplingcycle, a first phase of a conversion cycle, and a second phase of theconversion cycle, where the second phase includes a reset (RST)operation.

FIG. 13 is a diagram illustrating a voltage level of an amplifier outputand a voltage level of an MDAC output during the conversion cycle of theMDAC shown in FIG. 11.

FIG. 14 is a diagram illustrating an equivalent circuit of the MDACshown in FIG. 11 that operates during the first phase of the conversioncycle.

FIG. 15 is a diagram illustrating an equivalent circuit of the MDACshown in FIG. 11 that operates during the starting period of the secondphase of the conversion cycle.

FIG. 16 is a diagram illustrating an equivalent circuit of the MDACshown in FIG. 11 that operates during the remaining period of the secondphase of the conversion cycle.

FIG. 17 is a diagram illustrating a pipelined ADC according to anembodiment of the present invention.

FIG. 18 is a circuit diagram illustrating another MDAC with pre-samplingaccording to an embodiment of the present invention.

FIG. 19 is a circuit diagram of an MDAC with pre-sampling that utilizesa common-mode voltage as a pre-defined voltage according to anembodiment of the present invention.

FIG. 20 is a circuit diagram of an MDAC with pre-sampling that utilizesa different switch arrangement according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claims,which refer to particular components. As one skilled in the art willappreciate, electronic equipment manufacturers may refer to a componentby different names. This document does not intend to distinguish betweencomponents that differ in name but not in function. In the followingdescription and in the claims, the terms “include” and “comprise” areused in an open-ended fashion, and thus should be interpreted to mean“include, but not limited to . . . ”. Also, the term “couple” isintended to mean either an indirect or direct electrical connection.Accordingly, if one device is coupled to another device, that connectionmay be through a direct electrical connection, or through an indirectelectrical connection via other devices and connections.

FIG. 1 is a block diagram illustrating a multiplying digital-to-analogconverter (MDAC) with pre-sampling according to an embodiment of thepresent invention. The MDAC 100 includes an operational amplifier 102, aswitch circuit 104, and a plurality of capacitive circuits including apre-sampling capacitor circuit 106 and a sampling capacitor circuit 108.The operational amplifier 102 has an input port 112 and an output port114. For example, the input port 112 may include a non-inverting inputnode (+) and an inverting input node (−), and the output port 114 mayinclude a non-inverting output node (+) and an inverting output node(−). Since the MDAC 100 is a switched capacitor circuit, the switchcircuit 104 is arranged to control interconnection between theoperational amplifier 102, the sampling capacitor circuit 108, and thepre-sampling capacitor circuit 106.

An operation of the DAC-subtract-gain function performed by the MDAC 100may be divided into a sampling cycle and a conversion cycle followingthe sampling cycle. During the sampling cycle of the MDAC 100, theswitch circuit 104 is arranged to connect a pre-defined voltage V_(pd)to the pre-sampling capacitor circuit 106, connect a plurality ofreference voltages (e.g., V_(refn), V_(cm), and V_(refp), whereV_(refp)>V_(cm)>V_(refn) and V_(cm)=V_(refp)+V_(refn)=0V) to thepre-sampling capacitor circuit 106, disconnect the pre-samplingcapacitor circuit 106 from the input port 112 of the operationalamplifier 102, disconnect the pre-sampling capacitor circuit 106 fromthe sampling capacitor circuit 108, disconnect the output port 114 ofthe operational amplifier 102 from the sampling capacitor circuit 108,and connect a voltage input V_IN of the MDAC 100 to the samplingcapacitor circuit 108. For example, the voltage input V_IN may be adifferential input including a positive signal V_(ip) and a negativesignal V_(in) (i.e., V_IN=V_(ip)−V_(in)).

During the conversion cycle of the MDAC 100, the switch circuit 104 isarranged to connect the pre-sampling capacitor circuit 106 to thesampling capacitor circuit 108, where a configuration of connectionbetween the pre-sampling capacitor circuit 106 and the samplingcapacitor circuit 108 depends on a quantization result of the voltageinput V_IN, and the switch circuit 104 is further arranged to disconnectthe pre-defined voltage V_(pd) from the pre-sampling capacitor circuit106, disconnect the reference voltages (e.g., V_(refn), V_(cm), andV_(refp)) from the pre-sampling capacitor circuit 106, connect thepre-sampling capacitor circuit 106 to the input port 112 of theoperational amplifier 102, connect the output port 114 of theoperational amplifier 102 to the sampling capacitor circuit 108, anddisconnect the voltage input V_IN from the sampling capacitor circuit108. The use of the pre-sampling capacitor circuit 106 can achieveoperational amplifier power relaxation as well as reference buffer powerrelaxation.

FIG. 2 is a circuit diagram of an MDAC with pre-sampling according to anembodiment of the present invention. The MDAC 100 shown in FIG. 1 may beimplemented by the MDAC 200 shown in FIG. 2. The MDAC 200 includes anoperational amplifier OPAMP, a plurality of pre-sampling capacitorsC_(ps1), C_(ps0), C_(ps-1), C′_(ps1), C′_(ps0), C′_(ps-1), a pluralityof sampling capacitors C_(sam), C′_(sam), and a plurality of switchesSW1, SW2, SW3, SW4, SW5, SW6, SW7, SW8, SW9, SW10, SW11, SW′1, SW′2,SW′3, SW′4, SW′5, SW′6, SW′7, SW′8, SW′9, SW′10, SW′11. The operationalamplifier 102 shown in FIG. 1 may be implemented using the operationalamplifier OPAMP shown in FIG. 2, where the operational amplifier OPAMPis a differential amplifier having an input port consisting of anon-inverting input node (+) and an inverting input node (−) and anoutput port consisting of an inverting output node (−) and anon-inverting output node (+). The pre-sampling capacitor circuit 106shown in FIG. 1 may be implemented using the pre-sampling capacitorsC_(ps1), C_(ps0), C_(ps-1), C′_(ps1), C′_(ps0), C′_(ps-1) shown in FIG.2. The sampling capacitor circuit 108 shown in FIG. 1 may be implementedusing sampling capacitors C_(sam), C′_(sam) shown in FIG. 2. The switchcircuit 104 shown in FIG. 1 may be implemented using switches SW1-SW11,SW′1-SW′11 shown in FIG. 2.

The pre-sampling capacitors C_(ps1), C_(ps0), C_(ps-1), C′_(ps1),C′_(ps0), C′_(ps-1) are used to pre-sample V_(ref), 0V, and −V_(ref),where V_(refp)−V_(refn)=V_(ref), V_(refn)−V_(refp)=−V_(ref), andV_(em)=0V. The sampling capacitors C_(sam) and C′_(sam) are used tosample a voltage input (V_(ip)−V_(in)) that is a differential inputconsisting of a positive signal V_(ip) and a negative signal V_(in). Theprinciple of the proposed MDAC design with pre-sampling is to combinevoltage differences held by capacitors to achieve reference voltagesubtraction and input voltage amplification. FIG. 3 is a diagramillustrating the principle of the proposed MDAC design with pre-samplingaccording to an embodiment of the present invention. Suppose that onevoltage difference ΔV1 is held between a top plate and a bottom plate ofthe capacitor C1, and another voltage difference ΔV2 is held between atop plate and a bottom plate of the capacitor C2. When the capacitors C1and C2 are connected in series, the voltage across the series-connectedcapacitors C1 and C1 is equal to ΔV1+ΔV2. Regarding the MDAC 200, thevoltage output V_OUT acts as an MDAC output, and), and is determined byΔV1=2*(V_(ip)−V_(in)) and ΔV2=D_(out)*V_(ref), that is,V_OUT=2*(V_(ip)−V_(in))+D_(out)*V_(ref). The voltage 2*(V_(ip)−V_(in))is achieved via sampling capacitors. The voltage D_(out)*V_(ref) isachieved via pre-sampling capacitors and pre-sampling capacitorselection. FIG. 4 is a diagram illustrating a transfer curve of the MDAC200 shown in FIG. 2. If (V_(ip)−V_(in))>V_(ref)/4, D_(out)=+1 andV_OUT=2*(V_(ip)−V_(in))+V_(ref). If −V_(ref)/4 G (V_(ip)−V_(in))V_(ref)/4, D_(out)=0 and V_OUT=2*(V_(ip)−V_(in)). When(V_(ip)−V_(in))<−V_(ref)/4, D_(out)=−1 andV_OUT=2*(V_(ip)−V_(in))−V_(ref). Further details of the proposed MDAC200 with pre-sampling are described as below.

As mentioned above, an operation of the DAC-subtract-gain functionperformed by the MDAC 200 is divided into a sampling cycle and aconversion cycle following the sampling cycle. For example, the samplingcycle is enabled by a first clock, and the conversion cycle is enabledby a second clock, where the first clock and the second clock arenon-overlapping clocks, and on/off statuses of switches may becontrolled by the first clock and the second clock. The switch SW1 hasone node coupled to a pre-defined voltage (e.g., a bias voltage V_(bias)of the operational amplifier OPAMP) and another node coupled to oneplate of each of pre-sampling capacitors C_(ps1), C_(ps0), C_(ps-1). Inthis embodiment, V_(pd)=V_(bias). The switch SW′1 has one node coupledto the pre-defined voltage (e.g., bias voltage V_(bias) of theoperational amplifier OPAMP) and another node coupled to one plate ofeach of pre-sampling capacitors C′_(ps1), C′_(ps0), C′_(ps-1). Theswitch SW5 has one node coupled to the non-inverting input node (+) ofthe operational amplifier OPAMP and another node coupled to one plate ofeach of pre-sampling capacitors C_(ps1), C_(ps0), C_(ps-1). The switchSW′1 has one node coupled to the inverting input node (−) of theoperational amplifier OPAMP and another node coupled to one plate ofeach of pre-sampling capacitors C′_(ps1), C′_(ps0), C′_(ps-1). Theswitch SW9 has one node coupled to the inverting output node (−) of theoperational amplifier OPAMP and another node coupled to one plate of thesampling capacitor C_(sam). The switch SW′9 has one node coupled to thenon-inverting output node (+) of the operational amplifier OPAMP andanother node coupled to one plate of the sampling capacitor C′_(sam).

The switch SW2 has one node coupled to the reference voltage V_(refp),and another node coupled to another plate of the pre-sampling capacitorC_(ps1). The switch SW′2 has one node coupled to the reference voltageV_(refn), and another node coupled to another plate of the pre-samplingcapacitor C′_(ps1). The switch SW3 has one node coupled to the referencevoltage V_(cm), and another node coupled to another plate of thepre-sampling capacitor C_(ps0). The switch SW′3 has one node coupled tothe reference voltage V_(cm), and another node coupled to another plateof the pre-sampling capacitor C′_(ps0). The switch SW4 has one nodecoupled to the reference voltage V_(refn), and another node coupled toanother plate of the pre-sampling capacitor C_(ps-1). The switch SW′4has one node coupled to the reference voltage V_(refp), and another nodecoupled to another plate of the pre-sampling capacitor C′_(ps-1). Whenthe pair of pre-sampling capacitors C_(ps1) and C′_(ps1) is selected forproviding a pre-sampled reference voltage V_(ref) according to aquantization result of the voltage input, the pair of pre-samplingcapacitors C_(ps0) and C′_(ps0) and the pair of pre-sampling capacitorsC_(ps-1) and C′_(ps-1) are not selected.

The switch SW6 has one node coupled to another plate of the pre-samplingcapacitor C_(ps1) and another node coupled to another plate of thesampling capacitor C_(sam). The switch SW7 has one node coupled toanother plate of the pre-sampling capacitor C_(ps0) and another nodecoupled to another plate of the sampling capacitor C_(sam). The switchSW8 has one node coupled to another plate of the pre-sampling capacitorC_(ps-1) and another node coupled to another plate of the samplingcapacitor C_(sam). The switch SW′6 has one node coupled to another plateof the pre-sampling capacitor C′_(ps1) and another node coupled toanother plate of the sampling capacitor C′_(sam). The switch SW′7 hasone node coupled to another plate of the pre-sampling capacitor C′_(ps0)and another node coupled to another plate of the sampling capacitorC′_(sam) The switch SW′8 has one node coupled to another plate of thepre-sampling capacitor C′_(ps-1) and another node coupled to anotherplate of the sampling capacitor C′_(sam).

The switch SW10 has one node coupled to another plate of the samplingcapacitor C_(sam) and another node coupled to the negative signal V_(in)of the differential voltage input. The switch SW11 has one node coupledto one plate of the sampling capacitor C_(sam) and another node coupledto the positive signal V_(ip) of the differential voltage input. Theswitch SW′10 has one node coupled to another plate of the samplingcapacitor C′_(sam) and another node coupled to the positive signalV_(ip) of the differential voltage input. The switch SW′11 has one nodecoupled to one plate of the sampling capacitor C′_(sam) and another nodecoupled to the negative signal V_(in) of the differential voltage input.

During the sampling cycle, each of the switches SW1, SW2, SW3, SW4,SW10, SW11, SW′1, SW2′, SW′3, SW′4, SW′10, SW′11 is switched on, andeach of the switches SW5, SW6, SW7, SW8, SW9, SW′5, SW′6, SW′7, SW′8,SW′9 is switched off. FIG. 5 is a diagram illustrating an equivalentcircuit of the MDAC 200 operating during the sampling cycle. The voltagedifference (V_(refp)−V_(bias)) is held between two plates of thepre-sampling capacitor C_(ps1). The voltage difference (V_(cm)−V_(bias))is held between two plates of the pre-sampling capacitor C_(ps0). Thevoltage difference (V_(refn)−V_(bias)) is held between two plates of thepre-sampling capacitor C_(ps-1). The voltage difference(V_(refn)−V_(bias)) is held between two plates of the pre-samplingcapacitor C′_(ps1). The voltage difference (V_(cm)−V_(bias)) is heldbetween two plates of the pre-sampling capacitor C′_(ps0). The voltagedifference (V_(refp)−V_(bias)) is held between two plates of thepre-sampling capacitor C′_(ps-1). The voltage difference (V_(in)−V_(ip))is held between two plates of the sampling capacitor C_(sam). Thevoltage difference (V_(ip)−V_(in)) is held between two plates of thesampling capacitor C′_(sam).

During the conversion cycle, each of the switches SW1, SW2, SW3, SW4,SW10, SW11, SW′1, SW2′, SW′3, SW′4, SW′10, SW′11 is switched off, andeach of the switches SW5, SW9, SW′5, SW′9 is switched on. Regarding eachof switches SW6, SW7, SW8, SW′6, SW′7, SW′8, it is selectively switchedon in response to the quantization result of the voltage input(V_(ip)−V_(in)). When the pair of pre-sampling capacitors C_(ps1) andC′_(ps1) is selected for providing the pre-sampled reference voltageV_(ref) (i.e., V_(refp)−V_(refn)) according to the quantization resultof the voltage input (V_(ip)−V_(in)), the pair of pre-samplingcapacitors C_(ps0) and C′_(ps0) and the pair of pre-sampling capacitorsC_(ps-1) and C′_(ps-1) are not selected. When the pair of pre-samplingcapacitors C_(ps0) and C′_(ps0) is selected for providing thepre-sampled reference voltage 0V (i.e., V_(cm)−V_(cm)) according to thequantization result of the voltage input (V_(ip)−V_(in)), the pair ofpre-sampling capacitors C_(ps1) and C′_(ps1) and the pair ofpre-sampling capacitors C_(ps-1) and C′_(ps-1) are not selected. Whenthe pair of pre-sampling capacitors C_(ps-1) and C′_(ps-1) is selectedfor providing the pre-sampled reference voltage −V_(ref) (i.e.,V_(refn)−V_(refp)) according to the quantization result of the voltageinput (V_(ip)−V_(in)), the pair of pre-sampling capacitors C_(ps0) andC′_(ps0) and the pair of pre-sampling capacitors C_(ps1) and C′_(ps1)are not selected.

For example, when (V_(ip)−V_(in)) is larger than V_(ref)/4, thequantization result of the voltage input (V_(ip)−V_(in)) may generate a2-bit digital output “11”, and a decision circuit may refer to thequantization result to switch on the switches SW8 and SW′8 and switchoff the switches SW6, SW6′, SW7, and SW′7. FIG. 6 is a diagramillustrating an equivalent circuit of the MDAC 200 under a condition of(V_(ip)−V_(in))>V_(ref)/4 during the conversion cycle. As shown in FIG.6, the pre-sampling capacitor C_(ps-1) and the sampling capacitorC_(sam) are connected in series, and the pre-sampling capacitorC′_(ps-1) and the sampling capacitor C′_(sam) are connected in series,where one plate of the pre-sampling capacitor C_(ps-1) is connected to avirtual ground (floating ground) of the operational amplifier OPAMP, andone plate of the pre-sampling capacitor C′_(ps-1) is connected to avirtual ground (floating ground) of the operational amplifier OPAMP. Thevoltage difference between the inverting output node (−) and thenon-inverting input node (+) of the operational amplifier OPAMP is equalto (V_(ip)−V_(in))+(V_(refn)−V_(bias)). The voltage difference betweenthe non-inverting output node (+) and the inverting input node (−) ofthe operational amplifier OPAMP is equal to(V_(in)−V_(ip))+(V_(refp)−V_(bias)) Hence, the voltage output V_OUT canbe expressed as follows:

V_OUT=(V _(ip) −V _(in))+(V _(refn) −V _(bias))−[(V _(in) −V _(ip))+(V_(refp) −V _(bias))]=2*(V _(ip) −V _(in))+(V _(refn) −V _(refp))=2*(V_(ip) −V _(in))−V _(ref)

For another example, when (V_(ip)−V_(in)) is not larger than V_(ref)/4and is not smaller than −V_(ref)/4, the quantization result of thevoltage input (V_(ip)−V_(in)) may generate a 2-bit digital output “01”,and a decision circuit may refer to the quantization result to switch onthe switches SW7 and SW′7 and switch off the switches SW6, SW6′, SW8,and SW′8. FIG. 7 is a diagram illustrating an equivalent circuit of theMDAC 200 operating under a condition of −V_(ref)/4(V_(ip)−V_(in))<V_(ref)/4 during the conversion cycle. As shown in FIG.7, the pre-sampling capacitor C_(ps0) and the sampling capacitor C_(sam)are connected in series, and the pre-sampling capacitor C′_(ps0) and thesampling capacitor C′_(sam) are connected in series, where one plate ofthe pre-sampling capacitor C_(ps0) is connected to a virtual ground(floating ground) of the operational amplifier OPAMP, and one plate ofthe pre-sampling capacitor C′_(ps0) is connected to a virtual ground(floating ground) of the operational amplifier OPAMP. The voltagedifference between the inverting output node (−) and the non-invertinginput node (+) of the operational amplifier OPAMP is equal to(V_(ip)−V_(in))+(V_(cm)−V_(bias)). The voltage difference between thenon-inverting output node (+) and the inverting input node (−) of theoperational amplifier OPAMP is equal to(V_(in)−V_(ip))+(V_(cm)−V_(bias)). Hence, the voltage output V_OUT canbe expressed as follows:

V_OUT=(V _(ip) −V _(in))+(V _(cm) −V _(bias))−[(V _(in) −V _(ip))+(V_(cm) −V _(bias))]=2*(V _(ip) −V _(in))

For yet another example, when (V_(ip)−V_(in)) is smaller than−V_(ref)/4, the quantization result of the voltage input (V_(ip)−V_(in))may generate a 2-bit digital output “00”, and a decision circuit mayrefer to the quantization result to switch on the switches SW6 and SW′6and switch off the switches SW7, SW7′, SW8, and SW′8. FIG. 8 is adiagram illustrating an equivalent circuit of the MDAC 200 operatingunder a condition of −V_(ref)/4<(V_(ip)−V_(in)) during the conversioncycle. As shown in FIG. 8, the pre-sampling capacitor C_(ps1) and thesampling capacitor C_(sam) are connected in series, and the pre-samplingcapacitor C′_(ps1) and the sampling capacitor C′_(sam) are connected inseries, where one plate of the pre-sampling capacitor C_(ps1) isconnected to a virtual ground (floating ground) of the operationalamplifier OPAMP, and one plate of the pre-sampling capacitor C′_(ps1) isconnected to a virtual ground (floating ground) of the operationalamplifier OPAMP. The voltage difference between the inverting outputnode (−) and the non-inverting input node (+) of the operationalamplifier OPAMP is equal to (V_(ip)−V_(in))+(V_(refp)−V_(bias)). Thevoltage difference between the non-inverting output node (+) and theinverting input node (−) of the operational amplifier OPAMP is equal to(V_(in)−V_(ip))+(V_(refn)-V_(bias)) Hence, the voltage output V_OUT canbe expressed as follows:

V_OUT=(V _(ip) −V _(in))+(V _(refp) −V _(bias))−[(V _(in) −V _(ip))+(V_(refn) −V _(bias))]=2*(V _(ip) −V _(in))+(V _(refp) −V _(refn))=2*(V_(ip) −V _(in))+V _(ref)

Since one plate of the selected pre-sampling capacitor that receivesV_(bias) during the sampling cycle is floated during the conversioncycle, the operational amplifier OPAMP does not need to consume powerfor driving any capacitive load, and thus has power relaxation duringthe conversion cycle. In addition, as can be seen from FIGS. 6-8, thevoltage output V_OUT is derived from combining the voltage across theselected pre-sampling capacitor and the voltage across the samplingcapacitor. Hence, the operational amplifier OPAMP has a feedback factor(β) that is equal to 1. Compared to an operational amplifier with β<1,the operational amplifier OPAMP can have a wider bandwidth or lowerpower consumption. Moreover, since there is no charge flowing between aselected pre-charging capacitorC_(ps1)/C_(ps0)/C_(ps1)/C′_(ps1)/C′_(ps0)/C′_(ps1) and a samplingcapacitor C_(sam)/C′_(sam) during the conversion cycle, there is novoltage change at one plate of the selected pre-charging capacitorC_(ps1)/C_(ps0)/C_(ps-1)/C′_(ps1)/C′_(ps0)/C′_(ps-1) that receives areference voltage V_(rep)/V_(cm)/V_(refn) supplied from an externalreference buffer. Since no additional power is consumed by the referencebuffer to maintain the reference voltage V_(refp)/V_(cm)/V_(refn) duringthe conversion cycle, power relaxation of the reference buffer isachieved.

As mentioned above, the voltage output V_OUT is derived from combiningthe voltage across the selected pre-sampling capacitor and the voltageacross the sampling capacitor, where one plate of the selectedpre-charging capacitor receives a reference voltage supplied from anexternal reference buffer. The output common-mode voltage is notdetermined by the input common-mode voltage. Hence, the inputcommon-mode offset can be suppressed by the proposed MDAC 200 withpre-sampling. FIG. 9 is a diagram illustrating common-mode suppressionachieved by the proposed MDAC 200 with pre-sampling according to anembodiment of the present invention. Suppose that the positive signalV_(ip) of the differential voltage input has a common-mode offset (e.g.,10 mV), and the negative signal V_(in) of the differential voltage inputalso has a common-mode offset (e.g., 10 mV). During the sampling cycle,a fixed common-mode voltage is presented at the pre-sampling capacitor.During the conversion cycle, the pre-sampling capacitor and the samplingcapacitor are connected in series, where the voltage across theseries-connected pre-sampling capacitor and the sampling capacitor isequal to a summation of the voltage across the pre-sampling capacitorand the voltage across the sampling capacitor. Hence, the outputcommon-mode voltage is determined by the fixed common-mode voltageprovided by the pre-sampling capacitor, regardless of the inputcommon-mode offset (e.g., 10 mV).

A conventional differential amplifier may provide a tail current sourceas an effective technique for resolving problems associated withcommon-mode offset. However, the conventional differential amplifierwith the tail current source generally sacrifices speed for resolvingproblems associated with common-mode offset. Since the input common-modeoffset is suppressed by the proposed MDAC 200 with pre-sampling, theoperational amplifier OPAMP may be implemented by a differentialamplifier without a tail current source, as illustrated in FIG. 10. Forexample, the operational amplifier OPAMP may be a telescopicdifferential amplifier without a tail current source. Since no tailcurrent source is used by the operational amplifier OPAMP, the outputcurrent is no longer bounded by the tail current source. In this way,the operational amplifier OPAMP can operate at higher speed for settingthe differential amplifier output (e.g., OP_(out) generated from thetelescopic differential amplifier shown in FIG. 10) according to thedifferential amplifier input (e.g., {OP_(in1), OP_(ip1)} and {OP_(in0),OP_(ip0)} received by the telescopic differential amplifier shown inFIG. 10). Moreover, since the input common-mode offset can be suppressedby the proposed MDAC 200 with pre-sampling, the common-mode feedbackcircuit can be omitted from the operational amplifier OPAMP.

Compared to a conventional MDAC without pre-sampling, the proposed MDAC200 with pre-sampling selects one of −V_(ref), 0V, and V_(ref) viapre-sampling capacitor selection, achieves 2X voltage amplification bysampling the voltage input at the sampling capacitor only, has the samekT/C noise performance with the use of smaller sampling capacitor, usesan operational amplifier that has B=1, lower finite gain error and lowerpower consumption, and can use an operational amplifier with no tailcurrent source, and achieves power relaxation of a reference buffer thatis used to provide reference voltages V_(refp), V_(cm), and V_(refn).Furthermore, since the voltage output V_OUT is derived from combiningthe voltage across the selected pre-sampling capacitor and the voltageacross the sampling capacitor, no background calibration is needed bythe proposed MDAC 200 with pre-sampling.

Compared to a multi-stage operational amplifier, a single-stageoperational amplifier has lower power consumption and smaller outputswing. When the operational amplifier OPAMP is implemented by asingle-stage operational amplifier, the MDAC 200 with pre-sampling canbenefit from the low power consumption of the operational amplifierOPAMP. As mentioned above, the input common-mode offset can besuppressed by the proposed MDAC 200 with pre-sampling, and theoperational amplifier OPAMP may be implemented by a differentialamplifier without a tail current source. The operational amplifier OPAMPemployed by the proposed MDAC 200 with pre-sampling may be asingle-stage differential amplifier with no tail current source. Sincethe tail current source that affects the output swing is removed,single-stage differential amplifier with no tail current source canprovide the output swing required by the proposed MDAC 200 withpre-sampling.

In some embodiments of the present invention, acorrelated-level-shifting (CLS)-assisted operational amplifier may beused in an MDAC with pre-sampling to address the output swing issueencountered by the single-stage operational amplifier. It should benoted that using a CLS-assisted operational amplifier with/without tailcurrent in an MDAC with pre-sampling is optional. In practice, any MDACdesign using the proposed pre-sampling technique falls within the scopeof the present invention.

FIG. 11 is a circuit diagram of an MDAC with pre-sampling andCLS-assisted operational amplifier according to an embodiment of thepresent invention. The MDAC 100 shown in FIG. 1 may be implemented bythe MDAC 1100 shown in FIG. 11. In this embodiment, the operationalamplifier OPAMP may be implemented by a single-stage differentialamplifier with a tail current source or a single-stage differentialamplifier without a tail current source. The major difference betweenthe MDACs 200 and 1100 is that the MDAC 1100 further includes aplurality of CLS capacitors C_(CLS), C′_(CLS) and a plurality ofswitches SW12, SW′12, SW13. One plate of the CLS capacitor C_(CLS) iscoupled to one plate of the sampling capacitor C_(sam). One plate of theCLS capacitor C′_(CLS) is coupled to one plate of the sampling capacitorC′_(sam). The switch SW13 is a reset switch having one node coupled toanother plate of the CLS capacitor C_(CLS) and another node coupled toanother plate of the CLS capacitor C_(CLS). The switch SW12 has one nodecoupled to the inverting output node (−) of the operational amplifierOPAMP and another node coupled to one node of the switch SW13. Theswitch SW′12 has one node coupled to the non-inverting output node (+)of the operational amplifier OPAMP and another node coupled to anothernode of the switch SW13.

During the sampling cycle, each of the switches SW1, SW2, SW3, SW4,SW10, SW11, SW′1, SW2′, SW′3, SW′4, SW′10, SW′11 is switched on, andeach of the switches SW5, SW6, SW7, SW8, SW9, SW12, SW13, SW′5, SW′6,SW′7, SW′8, SW′9, SW′12 is switched off. During the conversion cycle,each of the switches SW1, SW2, SW3, SW4, SW10, SW11, SW′1, SW2′, SW′3,SW′4, SW′10, SW′11 is switched off, and each of the switches SW5, SW′5,SW9, SW′9 is switched on. Regarding each of switches SW6, SW7, SW8,SW′6, SW′7, and SW′8, it is selectively switched on in response to thequantization result of the voltage input (V_(ip)−V_(in)). Since theMDACs 200 and 1100 have the same control of the above switches duringthe sampling cycle and the conversion cycle, further description isomitted here for brevity.

Compared to the MDAC 200 with an amplifier output directly acting as theMDAC output (i.e., V_OUT), the MDAC 1100 does not use an amplifieroutput OP_OUT as the MDAC output (i.e., V_OUT). Specifically, theconversion cycle is divided into a first phase Amp1 and a second phaseAmp2 following the first phase Amp1. In addition, the second phase Amp2includes a starting period in which a reset (RST) operation occurs forquickly resetting the amplifier output OP_OUT. FIG. 12 is a diagramillustrating an operation of the DAC-subtract-gain function performed bythe MDAC 1100 includes a sampling cycle, a first phase Amp1 of aconversion cycle, and a second phase Amp2 of the conversion cycle, wherethe second phase Amp2 includes a reset (RST) operation.

Please refer to FIG. 11 in conjunction with FIG. 13. FIG. 13 is adiagram illustrating a voltage level of an amplifier output OP_OUT and avoltage level of an MDAC output (i.e., V_OUT) during the conversioncycle of the MDAC 1100. During the first phase Amp1 of the conversioncycle, each of the switches SW9 and SW′9 is switched on, and each of theswitches SW12, SW′12, SW13 is switched off. FIG. 14 is a diagramillustrating an equivalent circuit of the MDAC 1100 operating during thefirst phase Amp1 of the conversion cycle. Since the MDAC output iscoupled to the amplifier output via the switches SW9 and SW′9, thevoltage level of the amplifier output OP_OUT is substantially the sameas the voltage level of the MDAC output (i.e., V_OUT), as shown in FIG.13.

During the starting period of the second phase Amp2 of the conversioncycle, each of the switches SW9 and SW′9 is switched off, and each ofthe switches SW12, SW′12, SW13 is switched on. FIG. 15 is a diagramillustrating an equivalent circuit of the MDAC 1100 operating during thestarting period of the second phase Amp2 of the conversion cycle. Asshown in FIG. 13, the voltage level of the MDAC output (i.e., V_OUT) ismaintained by the CLS capacitors C′_(CLS) and C′_(CLS), while thevoltage level of the amplifier output OP_OUT is reset to the common-modevoltage (e.g., 0V).

During the remaining period of the second phase Amp2 of the conversioncycle, each of the switches SW9, SW′9, SW13 is switched off, and each ofthe switches SW12, SW′12 is switched on. FIG. 16 is a diagramillustrating an equivalent circuit of the MDAC 1100 operating during theremaining period of the second phase Amp2 of the conversion cycle. Asshown in FIG. 13, the operational amplifier OPAMP keeps adjusting theamplifier output OP_OUT, such that the voltage level of the MDAC output(i.e., V_OUT) is further adjusted by the voltage level of the amplifieroutput OP_OUT through capacitive coupling. As can be seen from FIG. 13,the amplification operation of the operational amplifier OPAMP isdivided into two phases Amp1 and Amp2. The output swing needed by theoperational amplifier OPAMP operating under the second phase Amp2 isreduced after the amplifier output OP_OUT is reset. In this way, theMDAC output (i.e., V_OUT) with a larger swing can be successfullyobtained by using a CLS-assisted single-stage amplifier with a smalleroutput swing. Moreover, with the help of the CLS capacitors and thetwo-step amplification, the finite gain error of the operationalamplifier OPAMP (i.e., CLS-assisted single-stage amplifier) can bereduced greatly.

The proposed MDAC with pre-sampling (or proposed MDAC with pre-samplingand CLS-assisted operational amplifier) may be employed by ananalog-to-digital converter (ADC), such as a pipelined ADC or atime-interleaved ADC using pipelined ADCs. FIG. 17 is a diagramillustrating a pipelined ADC according to an embodiment of the presentinvention. The pipelined ADC 1700 includes a plurality of stages1702_1-1702_N and a combining circuit 1704. The stages 1702_1-1702_N areconnected in a pipeline, and are arranged to generate a plurality ofdigital outputs D_1-D_N. The combining circuit 1704 is arranged tocombine the digital outputs D_1-D_N to generate a final digital output.The stage 1702_N is a terminal ADC. For example, the terminal ADC may beimplemented by a SAR ADC. In this embodiment, each of the stages1702_1-1702_(N−1) may employ the proposed MDAC with pre-sampling (orproposed MDAC with pre-sampling and CLS-assisted operational amplifier).Taking the stage 1702_1 for example, it includes a quantization circuit(QTZ) 1712, a decision circuit 1714, and an MDAC 1716. The quantizationcircuit 1712 generates a quantization result of a voltage input of thestage 1702_1, where the digital output D_1 of the stage 17021 depends onthe quantization result of the voltage input of the stage 1702_1. TheMDAC 1716 may be implemented by the MDAC 200/1100. The decision circuit1714 is arranged to select one of the pre-sampling capacitors C_(ps1),C_(ps0), C_(ps-1) that will be series-connected to one samplingcapacitor C_(sam) during the conversion cycle, and further select one ofthe pre-sampling capacitors C′_(ps1), C′_(ps0), C′_(ps-1) that will beseries-connected to another sampling capacitor C′_(sam) during theconversion cycle. For example, the decision circuit 1714 refers to thequantization result of the voltage input (e.g., digital output D_1) todetermine D_(out)*V_(ref) that will be combined with 2*(V_(ip)−V_(in)),where D_(out)=+1 if (V_(ip)−V_(in))<−V_(ref)/4, D_(out)=0 if −V_(ref)/4(V_(ip)−V_(in)) G V_(ref)/4, and D_(out)=−1 if(V_(ip)−V_(in))>V_(ref)/4.

Regarding the pipelined ADC 1700, one stage with an MDAC implemented byMDAC 200/1100 has a 1.5-bit/stage structure. However, this is forillustrative purposes only, and is not meant to be a limitation of thepresent invention. With proper modifications made to MDAC 200/1100, onestage with a modified MDAC may have a 2.5-bit/stage structure or a3.5-bit/stage structure. FIG. 18 is a circuit diagram illustratinganother MDAC with pre-sampling according to an embodiment of the presentinvention. Regarding the pipelined ADC 1700, one stage with an MDACimplemented by MDAC 1800 has a 2.5-bit/stage structure. The majordifference between MDACs 200 and 1800 is that the MDAC 1800 includesfour sampling capacitors C_(sam1), C_(sam2), C′_(sam1), C′_(sam2) andadditional switches SW14 and SW′14. Each of the switches SW10, SW11,SW14, SW10′, SW′11, SW′14 is switched on during the sampling cycle andis switched off during the conversion cycle. Since a person skilled inthe pertinent art can readily understand details and benefits of thepre-sampling technique employed by MDAC 1800 after reading aboveparagraphs directed to MDAC 200, further description is omitted here forbrevity.

In above embodiments shown in FIGS. 2, 11, and 18, the pre-definedvoltage V_(pd) is set by the bias voltage V_(bias) of the operationalamplifier OPAMP. However, these are for illustrative purposes only, andare not meant to be limitations of the present invention. Alternatively,the embodiments shown in FIGS. 2, 11, and 18 may be modified to have thepre-defined voltage V_(pd) set by a different voltage such as acommon-mode voltage (e.g., 0V). FIG. 19 is a circuit diagram of an MDACwith pre-sampling that utilizes a common-mode voltage as a pre-definedvoltage according to an embodiment of the present invention. The majordifference between the MDACs 200 and 1900 is that one node of the switchSW1 is arranged to receive the reference voltage V_(CM), and one node ofthe switch SW′1 is arranged to receive the reference voltage V_(CM),where the reference voltage V_(CM) is a common-mode voltage (e.g., 0V).

In above embodiments shown in FIGS. 2, 11, and 18, an arrangement ofswitches SW1, SW′1, SW5, SW′5 controls that one plate of each of thepre-sampling capacitors C_(ps1), C_(ps0), C_(ps-1), C′_(ps1), C′_(ps0),C′_(ps-1) is connected to the pre-defined voltage (e.g.,V_(pd)=V_(bias)) and is disconnected from the input port of theoperational amplifier OPAMP during the sampling cycle, and controls thatone plate of each of the pre-sampling capacitors C_(ps1), C_(ps0),C_(ps-1), C′_(ps1), C′_(ps0), C′_(ps-1) is disconnected from thepre-defined voltage (e.g., V_(pd)=V_(bias)) and is connected to theinput port of the operational amplifier OPAMP during the conversioncycle. However, these are for illustrative purposes only, and are notmeant to be limitations of the present invention. Alternatively, theembodiments shown in FIGS. 2, 11, and 18 may be modified to have adifferent switch arrangement that can achieve the same objective ofmaking one plate of each of the pre-sampling capacitors C_(ps1),C_(ps0), C_(ps-1), C′_(ps1), C′_(ps0), C′_(ps-1) connected to thepre-defined voltage (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) anddisconnected from the input port of the operational amplifier OPAMPduring the sampling cycle, and making one plate of each of thepre-sampling capacitors C_(ps1), C_(ps0), C_(ps-1), C′_(ps1), C′_(ps0),C′_(ps-1) disconnected from the pre-defined voltage (e.g.,V_(pd)=V_(bias) or V_(pd)=V_(cm)) and connected to the input port of theoperational amplifier OPAMP during the conversion cycle.

FIG. 20 is a circuit diagram of an MDAC with pre-sampling that utilizesa different switch arrangement according to an embodiment of the presentinvention. The major difference between the MDACs 200 and 2000 is thatthe switch SW1 is replaced by a switch group SG1 consisting of multipleswitches, the switch SW′1 is replaced by a switch group SG′1 consistingof multiple switches, the switch SW5 is replaced by a switch group SG5consisting of multiple switches, and the switch SW′5 is replaced by aswitch group SG′5 consisting of multiple switches.

The switch group SG1 has one switch with a first node coupled to thepre-defined voltage V_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) anda second node coupled to one plate of the pre-sampling capacitorC_(ps1), another switch with a first node coupled to the pre-definedvoltage V_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) and a secondnode coupled to one plate of the pre-sampling capacitor C_(ps0), and yetanother switch with a first node coupled to the pre-defined voltageV_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) and a second nodecoupled to one plate of the pre-sampling capacitor C_(ps-1).

The switch group SG′1 has one switch with a first node coupled to thepre-defined voltage V_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) anda second node coupled to one plate of the pre-sampling capacitorC′_(ps1), another switch with a first node coupled to the pre-definedvoltage V_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) and a secondnode coupled to one plate of the pre-sampling capacitor C′_(ps0), andyet another switch with a first node coupled to the pre-defined voltageV_(pd) (e.g., V_(pd)=V_(bias) or V_(pd)=V_(cm)) and a second nodecoupled to one plate of the pre-sampling capacitor C′_(ps-1).

The switch group SG5 has one switch with a first node coupled to thenon-inverting input node of the operational amplifier OPAMP and a secondnode coupled to one plate of the pre-sampling capacitor C_(ps1), anotherswitch with a first node coupled to the non-inverting input node of theoperational amplifier OPAMP and a second node coupled to one plate ofthe pre-sampling capacitor C_(ps0), and yet another switch with a firstnode coupled to the non-inverting input node of the operationalamplifier OPAMP and a second node coupled to one plate of thepre-sampling capacitor C_(ps-1).

The switch group SG′5 has one switch with a first node coupled to theinverting input node of the operational amplifier OPAMP and a secondnode coupled to one plate of the pre-sampling capacitor C′_(ps1),another switch with a first node coupled to the inverting input node ofthe operational amplifier OPAMP and a second node coupled to one plateof the pre-sampling capacitor C′_(ps0), and yet another switch with afirst node coupled to the inverting input node of the operationalamplifier OPAMP and a second node coupled to one plate of thepre-sampling capacitor C′_(ps-1).

During the sampling cycle of the MDAC 2000, all switches included in theswitch groups SG1 and SG′1 are switched on, and all switches included inthe switch groups SG5 and SG′5 are switched off. During the conversioncycle of the MDAC 2000, all switches included in the switch groups SG1and SG′1 are switched off, and all switches included in the switchgroups SG5 and SG′5 are switched on.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A multiplying digital-to-analog converter (MDAC)comprising: an operational amplifier, having an input port and an outputport; a sampling capacitor circuit; a pre-sampling capacitor circuit;and a switch circuit, arranged to control interconnection between theoperational amplifier, the sampling capacitor circuit, and thepre-sampling capacitor circuit; wherein during a sampling cycle of theMDAC, the switch circuit is arranged to connect a pre-defined voltage tothe pre-sampling capacitor circuit, connect a plurality of referencevoltages to the pre-sampling capacitor circuit, disconnect thepre-sampling capacitor circuit from the input port of the operationalamplifier, disconnect the pre-sampling capacitor circuit from thesampling capacitor circuit, disconnect the output port of theoperational amplifier from the sampling capacitor circuit, and connect avoltage input of the MDAC to the sampling capacitor circuit; and whereinduring a conversion cycle of the MDAC, the switch circuit is arranged toconnect the pre-sampling capacitor circuit to the sampling capacitorcircuit, where a configuration of connection between the pre-samplingcapacitor circuit and the sampling capacitor circuit depends on aquantization result of the voltage input, and is further arranged todisconnect the pre-defined voltage from the pre-sampling capacitorcircuit, disconnect said plurality of reference voltages from thepre-sampling capacitor circuit, connect the pre-sampling capacitorcircuit to the input port of the operational amplifier, connect theoutput port of the operational amplifier to the sampling capacitorcircuit, and disconnect the voltage input from the sampling capacitorcircuit.
 2. The MDAC of claim 1, wherein the pre-sampling capacitorcircuit comprises a first pre-sampling capacitor, a second pre-samplingcapacitor, a third pre-sampling capacitor, a fourth pre-samplingcapacitor, a fifth pre-sampling capacitor, and a sixth pre-samplingcapacitor; and the switch circuit comprises: a first switch, having afirst node coupled to the pre-defined voltage and a second node coupledto one plate of each of the first pre-sampling capacitor, the secondpre-sampling capacitor, and the third pre-sampling capacitor, whereinthe first switch is switched on during the sampling cycle and isswitched off during the conversion cycle; and a second switch, having afirst node coupled to the pre-defined voltage and a second node coupledto one plate of each of the fourth pre-sampling capacitor, the fifthpre-sampling capacitor, and the sixth pre-sampling capacitor, whereinthe second switch is switched on during the sampling cycle and isswitched off during the conversion cycle.
 3. The MDAC of claim 1,wherein the pre-sampling capacitor circuit comprises a firstpre-sampling capacitor, a second pre-sampling capacitor, a thirdpre-sampling capacitor, a fourth pre-sampling capacitor, a fifthpre-sampling capacitor, and a sixth pre-sampling capacitor; and theswitch circuit comprises: a first switch group, having one switch with afirst node coupled to the pre-defined voltage and a second node coupledto one plate of the first pre-sampling capacitor, another switch with afirst node coupled to the pre-defined voltage and a second node coupledto one plate of the second pre-sampling capacitor, and yet anotherswitch with a first node coupled to the pre-defined voltage and a secondnode coupled to one plate of the third pre-sampling capacitor; and asecond switch group, having one switch with a first node coupled to thepre-defined voltage and a second node coupled to one plate of the fourthpre-sampling capacitor, another switch with a first node coupled to thepre-defined voltage and a second node coupled to one plate of the fifthpre-sampling capacitor, and yet another switch with a first node coupledto the pre-defined voltage and a second node coupled to one plate of thesixth pre-sampling capacitor; wherein all switches included in the firstswitch group and the second switch group are switched on during thesampling cycle and are switched off during the conversion cycle.
 4. TheMDAC of claim 1, wherein the input port of the operational amplifiercomprises a non-inverting input node and an inverting input node; thepre-sampling capacitor circuit comprises a first pre-sampling capacitor,a second pre-sampling capacitor, a third pre-sampling capacitor, afourth pre-sampling capacitor, a fifth pre-sampling capacitor, and asixth pre-sampling capacitor; and the switch circuit comprises: a firstswitch, having a first node coupled to the non-inverting input node ofthe operational amplifier and a second node coupled to one plate of eachof the first pre-sampling capacitor, the second pre-sampling capacitor,and the third pre-sampling capacitor, wherein the first switch isswitched off during the sampling cycle and is switched on during theconversion cycle; and a second switch, having a first node coupled tothe inverting input node of the operational amplifier and a second nodecoupled to one plate of each of the fourth pre-sampling capacitor, thefifth pre-sampling capacitor, and the sixth pre-sampling capacitor,wherein the second switch is switched off during the sampling cycle andis switched on during the conversion cycle.
 5. The MDAC of claim 1,wherein the input port of the operational amplifier comprises anon-inverting input node and an inverting input node; the pre-samplingcapacitor circuit comprises a first pre-sampling capacitor, a secondpre-sampling capacitor, a third pre-sampling capacitor, a fourthpre-sampling capacitor, a fifth pre-sampling capacitor, and a sixthpre-sampling capacitor; and the switch circuit comprises: a first switchgroup, having one switch with a first node coupled to the non-invertinginput node of the operational amplifier and a second node coupled to oneplate of the first pre-sampling capacitor, another switch with a firstnode coupled to the non-inverting input node of the operationalamplifier and a second node coupled to one plate of the secondpre-sampling capacitor, and yet another switch with a first node coupledto the non-inverting input node of the operational amplifier and asecond node coupled to one plate of the third pre-sampling capacitor;and a second switch group, having one switch with a first node coupledto the inverting input node of the operational amplifier and a secondnode coupled to one plate of the fourth pre-sampling capacitor, anotherswitch with a first node coupled to the inverting input node of theoperational amplifier and a second node coupled to one plate of thefifth pre-sampling capacitor, and yet another switch with a first nodecoupled to the inverting input node of the operational amplifier and asecond node coupled to one plate of the sixth pre-sampling capacitor;wherein all switches included in the first switch group and the secondswitch group are switched off during the sampling cycle and are switchedon during the conversion cycle.
 6. The MDAC of claim 1, wherein saidplurality of reference voltages comprise a first reference voltage, asecond reference voltage, and a third reference voltage; thepre-sampling capacitor circuit comprises a first pre-sampling capacitor,a second pre-sampling capacitor, a third pre-sampling capacitor, afourth pre-sampling capacitor, a fifth pre-sampling capacitor, and asixth pre-sampling capacitor, where first plates of the firstpre-sampling capacitor, the second pre-sampling capacitor, and the thirdpre-sampling capacitor are coupled to one another, and first plates ofthe fourth pre-sampling capacitor, the fifth pre-sampling capacitor, andthe sixth pre-sampling capacitor are coupled to one another; and theswitch circuit comprises: a first switch, having a first node coupled tothe first reference voltage and a second node coupled to a second plateof the first pre-sampling capacitor, wherein the first switch isswitched on during the sampling cycle and is switched off during theconversion cycle; a second switch, having a first node coupled to thesecond reference voltage and a second node coupled to a second plate ofthe second pre-sampling capacitor, wherein the second switch is switchedon during the sampling cycle and is switched off during the conversioncycle; a third switch, having a first node coupled to the thirdreference voltage and a second node coupled to a second plate of thethird pre-sampling capacitor, wherein the third switch is switched onduring the sampling cycle and is switched off during the conversioncycle; a fourth switch, having a first node coupled to the thirdreference voltage and a second node coupled to a second plate of thefourth pre-sampling capacitor, wherein the fourth switch is switched onduring the sampling cycle and is switched off during the conversioncycle; a fifth switch, having a first node coupled to the secondreference voltage and a second node coupled to a second plate of thefifth pre-sampling capacitor, wherein the fifth switch is switched onduring the sampling cycle and is switched off during the conversioncycle; and a sixth switch, having a first node coupled to the firstreference voltage and a second node coupled to a second plate of thesixth pre-sampling capacitor, wherein the sixth switch is switched onduring the sampling cycle and is switched off during the conversioncycle.
 7. The MDAC of claim 1, wherein the sampling capacitor circuitcomprises a first sampling capacitor and a second sampling capacitor;the pre-sampling capacitor circuit comprises a first pre-samplingcapacitor, a second pre-sampling capacitor, a third pre-samplingcapacitor, a fourth pre-sampling capacitor, a fifth pre-samplingcapacitor, and a sixth pre-sampling capacitor, where first plates of thefirst pre-sampling capacitor, the second pre-sampling capacitor, and thethird pre-sampling capacitor are coupled to one another, and firstplates of the fourth pre-sampling capacitor, the fifth pre-samplingcapacitor, and the sixth pre-sampling capacitor are coupled to oneanother; and the switch circuit comprises: a first switch, having afirst node coupled to a second plate of the first pre-sampling capacitorand a second node coupled to one plate of the first sampling capacitor,wherein the first switch is switched off during the sampling cycle, andis selectively switched on in response to the quantization result of thevoltage input during the conversion cycle; a second switch, having afirst node coupled to a second plate of the second pre-samplingcapacitor and a second node coupled to said one plate of the firstsampling capacitor, wherein the second switch is switched off during thesampling cycle, and is selectively switched on in response to thequantization result of the voltage input during the conversion cycle; athird switch, having a first node coupled to a second plate of the thirdpre-sampling capacitor and a second node coupled to said one plate ofthe first sampling capacitor, wherein the third switch is switched offduring the sampling cycle, and is selectively switched on in response tothe quantization result of the voltage input during the conversioncycle; a fourth switch, having a first node coupled to a second plate ofthe fourth pre-sampling capacitor and a second node coupled to one plateof the second sampling capacitor, wherein the fourth switch is switchedoff during the sampling cycle, and is selectively switched on inresponse to the quantization result of the voltage input during theconversion cycle; a fifth switch, having a first node coupled to asecond plate of the fifth pre-sampling capacitor and a second nodecoupled to said one plate of the second sampling capacitor, wherein thefifth switch is switched off during the sampling cycle, and isselectively switched on in response to the quantization result of thevoltage input during the conversion cycle; a sixth switch, having afirst node coupled to a second plate of the sixth pre-sampling capacitorand a second node coupled to said one plate of the second samplingcapacitor, wherein the sixth switch is switched off during the samplingcycle, and is selectively switched on in response to the quantizationresult of the voltage input during the conversion cycle.
 8. The MDAC ofclaim 1, wherein the output port of the operational amplifier comprisesa non-inverting output node and an inverting output node; the samplingcapacitor circuit comprises a first sampling capacitor and a secondsampling capacitor; and the switch circuit comprises: a first switch,having a first node coupled to the inverting output node of theoperational amplifier and a second node coupled to one plate of thefirst sampling capacitor, wherein the first switch is switched offduring the sampling cycle and is switched on during the conversioncycle; and a second switch, having a first node coupled to thenon-inverting output node of the operational amplifier and a second nodecoupled to one plate of the second sampling capacitor, wherein thesecond switch is switched off during the sampling cycle and is switchedon during the conversion cycle.
 9. The MDAC of claim 1, wherein thevoltage input is a differential input comprising a positive signal and anegative signal; the sampling capacitor circuit comprises a firstsampling capacitor and a second sampling capacitor; and the switchcircuit comprises: a first switch, having a first node coupled to thenegative signal and a second node coupled to a first plate of the firstsampling capacitor, wherein the first switch is switched on during thesampling cycle and is switched off during the conversion cycle; a secondswitch, having a first node coupled to the positive signal and a secondnode coupled to a second plate of the first sampling capacitor, whereinthe second switch is switched on during the sampling cycle and isswitched off during the conversion cycle; a third switch, having a firstnode coupled to the positive signal and a second node coupled to a firstplate of the second sampling capacitor, wherein the third switch isswitched on during the sampling cycle and is switched off during theconversion cycle; and a fourth switch, having a first node coupled tothe negative signal and a second node coupled to a second plate of thesecond sampling capacitor, wherein the fourth switch is switched onduring the sampling cycle and is switched off during the conversioncycle.
 10. The MDAC of claim 1, wherein the operational amplifier is asingle-stage differential amplifier with no tail current source.
 11. TheMDAC of claim 1, further comprising: a first correlated-level-shifting(CLS) capacitor; and a second CLS capacitor; wherein the output port ofthe operational amplifier comprises a non-inverting output node and aninverting output node; the sampling capacitor circuit comprises a firstsampling capacitor and a second sampling capacitor; one plate of thefirst sampling capacitor is coupled to a first plate of the first CLScapacitor; one plate of the second sampling capacitor is coupled to afirst plate of the second CLS capacitor; and the switch circuitcomprises: a first switch, having a first node coupled to said one plateof the first sampling capacitor and a second node coupled to theinverting output node of the operational amplifier, wherein the firstswitch is switched on during a first phase of the conversion cycle, andis switched off during a second phase of the conversion cycle; a secondswitch, having a first node coupled to said one plate of the secondsampling capacitor and a second node coupled to the non-inverting outputnode of the operational amplifier, wherein the second switch is switchedon during the first phase of the conversion cycle, and is switched offduring the second phase of the conversion cycle; a third switch, havinga first node coupled to a second plate of the first CLS capacitor and asecond node coupled to a second plate of the second CLS capacitor,wherein the third switch is switched on during a starting period of thesecond phase of the conversion cycle, and is switched off during aremaining period of the second phase of the conversion cycle; a fourthswitch, having a first node coupled to the inverting node of theoperational amplifier and a second node coupled to the first node of thethird switch, wherein the fourth switch is switched off during the firstphase of the conversion cycle, and is switched on during the secondphase of the conversion cycle; and a fifth switch, having a first nodecoupled to the non-inverting node of the operational amplifier and asecond node coupled to the second node of the third switch, wherein thefifth switch is switched off during the first phase of the conversioncycle, and is switched on during the second phase of the conversioncycle.
 12. The MDAC of claim 11, wherein during the second phase of theconversion cycle, a voltage difference between said one plate of thefirst sampling capacitor and said one plate of the second samplingcapacitor acts as an MDAC output.
 13. The MDAC of claim 1, wherein thepre-defined voltage is a bias voltage of the operational amplifier. 14.The MDAC of claim 1, wherein the pre-defined voltage is one of thereference voltages.
 15. A pipelined analog-to-digital converter (ADC)comprising: a plurality of stages, connected in a pipeline and arrangedto generate a plurality of digital outputs, respectively; and acombining circuit, arranged to combine said plurality of digitaloutputs; wherein at least one of said plurality of stages comprises: aquantization circuit, arranged to generate a quantization result of avoltage input of said at least one of said plurality of stages, whereina digital output of said at least one of said plurality of stagesdepends on the quantization result of the voltage input; and amultiplying digital-to-analog converter (MDAC), comprising: anoperational amplifier, having an input port and an output port; asampling capacitor circuit; a pre-sampling capacitor circuit; and aswitch circuit, arranged to control interconnection between theoperational amplifier, the sampling capacitor circuit, and thepre-sampling capacitor circuit; wherein during a sampling cycle of theMDAC, the switch circuit is arranged to connect a pre-defined voltage tothe pre-sampling capacitor circuit, connect a plurality of referencevoltages to the pre-sampling capacitor circuit, disconnect thepre-sampling capacitor circuit from the input port of the operationalamplifier, disconnect the pre-sampling capacitor circuit from thesampling capacitor circuit, disconnect the output port of theoperational amplifier from the sampling capacitor circuit, and connect avoltage input of the MDAC to the sampling capacitor circuit; and whereinduring a conversion cycle of the MDAC, the switch circuit is arranged toconnect the pre-sampling capacitor circuit to the sampling capacitorcircuit, where a configuration of connection between the pre-samplingcapacitor circuit and the sampling capacitor circuit depends on thequantization result of the voltage input, and is further arranged todisconnect the pre-defined voltage from the pre-sampling capacitorcircuit, disconnect said plurality of reference voltages from thepre-sampling capacitor circuit, connect the pre-sampling capacitorcircuit to the input port of the operational amplifier, connect theoutput port of the operational amplifier to the sampling capacitorcircuit, and disconnect the voltage input from the sampling capacitorcircuit.